ATLAS gets its own luminosity detector

During the winter shutdown, the ATLAS collaboration has completed the installation of ALFA, the detector system that aims at the LHC absolute luminosity at Point 1 analysing the elastic scattering of protons at small angles.


Upper and lower ALFA Roman Pots as installed in sector 8-1 of the LHC tunnel, 240 metres from the ATLAS Interaction Point.

The detectors of the ALFA system are installed at ± 240 meters from the interaction point 1, on either side of the ATLAS detector. The whole system consists of four stations, two on each side of the interaction point. Each station is equipped with two Roman Pots; each pot – that is separated from the vacuum of the accelerator by a thin window but is connected with bellows to the beam-pipe – can be moved very close to the beam. “The Roman Pot technique has been used successfully in the past for the measurement of elastic scattering very close to the circulating beam,” says Patrick Fassnacht, Technical Coordinator of the project. “The ATLAS Roman Pots have been designed to move the detectors as close as 1 mm to the beam, but only from above and below, due to the mechanical constraints imposed by the two horizontal beam-pipes of the LHC.” The set-up is very similar to the one of TOTEM at point 5 of the LHC ring but with fewer pots.

The detectors are housed in the ALFA Roman Pots; the detector consists of scintillating fibers that ensure a very good spatial resolution of ~30 microns and a robust technical design. “Inserting the detectors into the pots requires a careful precision work due to the small margins and the tight working space available,” says Thomas Schneider from the PH/DT group who together with Sune Jacobsen, PhD student from the Niels Bohr Institute in Copenhagen, was responsible for the insertion. “Special tooling was prepared and insertions were exercised several times at the surface. This good preparation paid off; the insertion into the tunnel went very smoothly.”

The installation work has involved many people both from ATLAS participating Universities and from various CERN Departments. “Particularly critical was the bake-out of the equipment after the installation of the stations into the beam lines, but eventually everything went well,” said Anne-Laure Perrot, in charge of the experiment interface with the LHC machine.

The ALFA system, with its 30 physicists and engineers from over 8 institutes, is now getting its very first physics data after a final commissioning period with the beam, which has just started.



by CERN Bulletin